The promised performance of wide band gap electronic devices (e.g. GaN based) will result in much high power dissipation and localized heat generation at contacts and in channel regions than can be accommodated by current state-of-the-art thermal management configurations. As a consequence, use of conventional cooling techniques imposes a ceiling on wide band gap device performance and reliability. Overcoming such barriers requires thermal engineering at the macro, micro, and nano-scale, which can provide significant reductions in the near-junction temperature rise and component thermal resistance.
Specific challenges relate to heat spreading in certain types of radio frequency (rf) power devices. In such devices the local power densities can exceed 1 MW/cm2. Spreading this heat and lowering the junction temperature enables increased reliability and also continuous wave performance. In addition to electronic device applications, there is also a need to improve upon current state-of-the-art thermal management configurations in certain extreme optical applications.
Synthetic diamond materials have been proposed as an ideal solution in extreme thermal management applications due to the high in-plane thermal conductivity of such materials. For example, various grades of synthetic diamond material grown by chemical vapour deposition (CVD) are already commercially available for thermal heat spreading applications including both polycrystalline and single crystal synthetic diamond materials.
The thermal performance of a particular synthetic diamond material will depend on its macro, micro, and nano-scale structure. Factors that contribute to thermal performance are those that lead to scattering of phonons within the synthetic diamond material [J. E. Graebner, Diamond Films Technol., (Japan) 3 (1993) p 77 includes a survey of phonon scattering in diamond thin films]. For example in synthetic diamond materials factors which lead to scattering of phonons include: intrinsic mechanisms (phonon-phonon related); point defects (e.g. defects such as nitrogen and vacancy clusters); and extended defects (e.g. stacking faults and dislocations). As such, synthetic diamond materials which are optimized for improved thermal performance are those which have reduced defects in terms of both point defects and extended defects. Furthermore, synthetic diamond materials which are optimized for improved thermal performance may also be tailored to reduce intrinsic phonon scattering mechanisms.
Dominant amongst intrinsic phonon scattering mechanisms are those related to the relative masses of 12C and 13C. The natural abundance of 13C is 1.1% meaning that approximately 1 in every 100 atoms has a 12/13 difference in mass and hence different phonon energy. Isotopically controlled single crystal diamond theory [R. Berman, Thermal Conductivity in Solids (Clarendon Press 1976)] and experiment [e.g. General Electric, L. Wei, P. K. Kuo, R. L. Thomas, T. R Anthony, W. F. Banholzer, Phys Rev Lett 70 (1993) p 3764] has shown that bulk thermal conductivity can increase by nearly a factor of two up to 4000 W/mK. As such, it is known in the art that reducing the 13C content in synthetic diamond materials can reduce intrinsic phonon scattering and increase bulk thermal conductivity, particularly in relation to single crystal synthetic diamond materials. However, one problem with this approach is that such isotopically purified synthetic diamond materials require a fabrication process which utilizes an isotopically purified carbon source. Such isotopically purified carbon sources are expensive and thus while isotopically purified synthetic diamond materials can have improved thermal performance this improvement can be off-set by increased expense resulting in the materials having a reduced commercial viability in certain applications.
U.S. Pat. No. 6,582,513 (Apollo) also recognizes that the thermal conductivity of synthetic diamond materials can be increased by reducing the 13C content in such materials. This document also suggests that providing alternating layers of single crystal diamond material having different levels of dopant distributed throughout the single crystal lattice can be used to manage strain through the single crystal lattice due to lattice mismatches between differently doped diamond layers. Various examples are given including using alternating layers of undoped 12C and 13C diamond material. It is indicated that such a structure can end in either a 12C or a 13C layer and then be used to grow single crystal plates of either 12C or 13C diamond. However, this approach still requires a relatively larger quantity of isotopically purified carbon source material increasing cost and resulting in the materials having a reduced commercial viability in certain applications.
It may also be noted that isotopically purified synthetic diamond materials have also been proposed for use in non-thermal applications such as quantum sensing and quantum information processing. For example, WO2010010352 describes that fabricating isotopically purified synthetic diamond materials can improve the optical stability of certain fluorescent point defects disposed within synthetic diamond materials leading to performance improvements in quantum optics applications. Such quantum grade synthetic diamond materials are of the highest chemical purity and crystallographic quality and are much too expensive for use in more basic thermal heat spreading applications.
It is an aim of certain embodiments of the present invention to provide synthetic diamond materials which have improved thermal performance without significantly increasing fabrication costs leading to more commercial viable products for extreme thermal management applications.